If a worshipper intends to pray four rak'ahs of the Sunnah prayer before Asr with a single taslim, then the congregational Asr prayer is established after two rak'ahs, what is better for him: should he interrupt his prayer and join the congregation, and how does he interrupt his prayer? Or should he complete his prayer then join the congregation?

It is preferable to pray two rak'ahs before Asr with two salutations, because the Prophet (peace be upon him) did so. As for someone who intends to pray four [rak'ahs] and then the congregational prayer is called, the majority of scholars believe that he should complete the supererogatory prayer, unless he fears missing the congregation. And whoever fears missing the congregation should cut short the supererogatory prayer.